In this type of optical measuring device known as a confocal microscope, a test object is illuminated, for example, through a microscope objective by a patch of light. The patch of light is situated in the focal plane of the microscope objective. This plane, the so-called focal plane of the microscope objective, is once again imaged through the microscope objective into an image plane where an aperture (pinhole) is located. Situated behind this detection aperture is a photodiode, or image recorder.
For purposes of 3D-measurement, the object surface is moved (scanned) in the depth axis (z-axis) through the focal plane. If the object surface lies in the focal plane, then the patch of light is imaged sharply onto the detection aperture, and the photodiode measures a high intensity signal. If, on the other hand, the surface lies outside of the focal plane, then an unsharp image of the patch of light is formed on the detection aperture, and the photodiode measures a weak intensity signal. Various methods have been established for moving the test object through the focal plane for purposes of 3D measurement. For example, in a commercial confocal microscope, the test component is placed on a piezo-unit that performs the depth scanning, or specific components, together with the microscope objective, perform the depth scanning. Heretofore, confocal microscopy has been predominantly used for measuring planar objects or in the field of biology, as is shown in S. W. Paddock, “Confocal Laser Scanning Microscopy,” Bio Techniques, Vol. 27, No. 5, pp. 992-1004, November 1999.
A further measuring device of this type is configured as an autofocus system. In this context, just as in the aforementioned confocal microscope, the test object in the observed area is illuminated by a patch of light via an illumination optics and a measuring optics. The focal plane is once again supplied via the measuring optics to the detection unit, in which an astigmatic optics is arranged for producing an image on the image recorder. The image, as a function of the position of the observed area relative to the focal plane, has varying local distribution patterns of intensity, from which using a locally resolving image recorder the position of the observed area can be determined. Heretofore, autofocus systems of this type have been used in cameras for automatic sharp focussing or as distance sensors in CD-playing devices. For the design of an autofocus system, reference is made to Naumann/Schröder: Bauelemente der Optik, Carl Hanser Printing House, Munich, Vienna, 6th Edition, p. 349, and to Isailovic': Videodisc and Optical Memory Systems, Prentice-Hall Inc., 1985.
If the test object is illuminated by a patch of light, it must then still be scanned in the xy-direction. For this, various related-art techniques have also become widely accepted, such as beam folding using oscillating mirrors, moving the object in the xy-direction relative to the measuring system. When a Nipkow disk and a CCD camera are used, areal measurements of the objective surface can likewise be taken.
An alternative measuring device is configured as a Foucault laser. In this context, a parallel light beam is generated by a laser. This beam is focused via an optics (objective optics). In the illumination light path, an aperture (Foucault knife edge) is used, so that the laser beam cannot spread out in passing through the entire opening of the objective optics. The focused light is reflected back by the object and can pass, in the detection light path, through the full opening of the objective optics. Via a beam splitter, which is situated between the objective optics and the Foucault knife edge, the reflected light via a second optics (detection optics) is conveyed onto a locally resolving image recorder. By using the Foucault knife edge in the illumination light path, the image of the focus from the surface of the object has varying distribution patterns of intensity as a function of the position of the observed area of the object surface relative to the focal plane of the objective optics. From this varying local distribution of intensity, which is established and evaluated using the image recorder (e.g., CCD camera, four-quadrant diode, differential diode, etc.), the position of the observed area of the surface can be determined. Because the object is illuminated using only one patch of light, it must still be scanned in the xy-direction. For this, various techniques have established themselves in accordance with the related art (e.g., moving the object relative to the light beam, beam folding using oscillating mirrors, etc.).
If measurements are to be taken, for example, in narrow cavities, then difficulties arise when a confocal microscope is used.
In contrast to interferometric optical measuring devices in narrow cavities, e.g., as in white light interferometry, where it is necessary to assemble a relatively expensive reference arm having properties nearly identical to those of the object arm, a confocal microscope has a simple and cost-effective design.
The underlying object of the present invention is to devise an optical measuring device of the type mentioned at the outset which will enable surface measurements of a test object to be taken even at poorly accessible locations, such as in narrow cavities.